The present application claims priority from U.S. 60/749,374 filed Dec. 12, 2005. This referenced application is to be incorporated herein in its entirety.
The invention relates to the inspection of optical elements with a view to contamination, in particular to the determination of spatial distribution of contamination on the surface of optical elements, as well as to the determination of the composition of said contamination, with an examination system, an examination device and a manipulator. The investigated optical elements form part of an optical device, in particular of laser machining apparatus or projection illumination apparatus for microlithography. The invention also relates to an optical device that comprises such examination systems and examination devices or manipulators. Furthermore, the invention relates to a method for removing contamination from the surface of an optical element, as well as to a method for determining the thickness of contamination layers on optical elements.
Optical devices, in particular systems in which short-wave radiation is used, such as e.g. projection illumination apparatus for lithography optics, or systems for inspecting photomasks and wafers, as well as laser systems e.g. for laser machining apparatus, in particular UV laser systems and systems for guiding their beams, are associated with the danger that the surfaces of some of their optical elements become contaminated as a result of the atmosphere surrounding them.
Within the context of this patent application the term “atmosphere” refers to ambient air (including filtered or conditioned air), purge gases, immersion fluids and any impurities contained therein, as well as in the case of optical elements that are arranged in vacuum chambers the residual gases contained therein. The term “short-wave radiation” refers to UV light with wavelengths of less than 400 nm, in particular wavelengths of approximately 365 nm, 248 nm, 193 nm and 157 nm, as well as soft X-ray radiation, in particular in the region of 13.5 nm. The term “optical element” refers to a lens, mirror, base plate, grid or any other optical element.
Within the context of this patent application, the term “laser machining apparatus” relates to a material processing apparatus, in particular to a laser annealing apparatus which is useful in annealing of large substrates, in the field of flat panel display, such as liquid crystal display, or luminescence display manufacturing processes as well as in the fabrication of thin film photovoltaic devices. Such an apparatus allows for crystallization procedures such as excimer laser crystallization (ELC), sequential lateral solidification (SLS) or thin beam crystallization procedure (TDX). In particular, such an apparatus is useful in order to crystallize amorphous Silicon (a-Si) films forming polycrystalline Silicon (p-Si). Such polycrystalline Silicon thin films are widely used in microelectronics and display techniques as mentioned above. P-Si has a higher charge carrier mobility as compared to a-Si which is useful for the fabrication of higher speed switching or integration of higher quality driver electronics on the display substrate. Furthermore, p-Si has a lower absorption coefficient for light in the visual spectral range enabling p-Si to be used as a rear electrode for LCD-applications allowing backlight to be transmitted. Lastly, the defect density of p-Si is lower as compared with a-Si which is a prerequisite for the fabrication of high efficient solar cells. The conversion of a-Si into p-Si may be employed by heat treatment at around 1000° C. Such a procedure may only be used for a-Si on heat resistant substrates such as quartz. Such materials are expensive compared to normal float glass for display purposes. Light induced crystallization of a-Si allows the formation of p-Si from a-Si without destroying the substrate by the thermal load during crystallization. Amorphous Silicon may be deposited by a low cost process such as sputtering or chemical vapour deposition (CVD) on substrates such as glass, quartz or synthetics.
It is well known that contamination of optics by trace gases in the ambient atmosphere can have various causes so that contamination in its spatial arrangement and in its composition can differ considerably. Despite different characteristics in relation to the topography and/or chemical composition, contamination has at least in part the same effect on those optical characteristics of the optical system, which characteristics are demanded by the user to serve the intended purpose.
A typical example of the above relates to scattered light in an imaging optics system. Light that does not contribute to imaging but instead impinges upon the image plane at some other point is generally referred to as scattered light. For example in the case of projection illumination apparatus for the semiconductor industry, scattered light is an important criterion for the usability of the optical system. Usually, scattered light is regularly measured and monitored by the projection illumination apparatus at the system plane (i.e. at the location of the wafer), wherein an increase in scattered light indicates the presence of contamination.
However, the known methods for measuring scattered light in an optical system cannot unequivocally locate the source of the scattered light, wherein e.g. the depositing of material on at least one optical surface of the optical system is considered to be one possible source. Depending on the type of contamination (which in turn depends on the ambient atmosphere) the topography and chemical composition of this material can however be quite different, as described above.
Known materials that deposit on the optical surfaces include in particular: salts, in particular sulphates and phosphates, as well as chlorides, nitrites and nitrates with ammonium- or alkali- or alkaline earth metals as counter-ions. Furthermore, the depositing of thin films comprising hydrocarbons, as well as films comprising polymer materials rich in hydrocarbons, are known, as is the depositing of layers of polymeric Si compounds that arise as a result of the effect of light on siloxanes.
Apart from deposits, scattered light can also result from changes in the surface of the optical elements, e.g. by the etching of optical elements and/or of layers thereon, or by the chipping off of layers on optical elements. Furthermore, scattering centres can form in the material of the optical elements, e.g. as a result of blistering, changes in the homogeneity, formation of microchannels or microcracks.
US 2005/0094115 A1 describes projection illumination apparatus with an examination device for examining an optical element for contamination. The examination device comprises a light generation unit that radiates light onto the optical element, a photodetector that detects light transmitted or reflected by the optical element, as well as a processing device, connected to the photodetector, for processing the data measured by the detector. Preferably, by means of the examination system the reflectivity or the transmissivity of the optical elements is measured and compared to desired values. Projection illumination apparatus described therein further comprises devices for removing contamination from optical elements.
From DE 103 32 110 A1 of the applicant, furthermore a device for scattered-light inspection of optical elements is known, which device at least in part can be accommodated in a reticle holder or substrate holder of projection illumination apparatus for microlithography, and to this effect is accommodated in a suitable housing.
From DE 102 10 209 A1 by the applicant, a method and a device for scattered light inspection of transparent optical elements, in particular of blanks for optics components are known, in which method and device an examination light beam is guided through the optical element, and the scattered light is recorded that originates from a volume region of the optical element, through which volume region the examination light beam travels. In this arrangement, by means of a movable mirror, the examination light beam can be guided in a scanning manner along the entire extent of the optical element, and/or the specimen to be examined is mounted on an x-y translational table and is moved in a suitable manner so that a two-dimensional scattered-light map of the specimen to be examined can be produced.